PIP: Following a brief introduction to the history of AIDS and the global impact of the AIDS epidemic, the important, yet difficult to research role of perinatally transmitted human immunodeficiency virus (HIV) from mother to infant is pointed out. Approximately 80% of all HIV infections in children in the US and Europe stem from perinatal transmission; similar rates are expected for sub-Saharan Africa and the Caribbean. Accordingly, current understanding of the epidemiology of HIV transmission from mothers to children is reviewed. While obstacles exist to AIDS surveillance, determining the frequency and timing of perinatal HIV transmission, planning treatment trials, and clinical treatment, information has been gained over the years from AIDS surveillance data, HIV seroprevalence surveys, and studies of HIV transmission and disease progression. The focus here is primarily upon information obtained since early 1988, methodological issues, and future research priorities. The scope of the HIV epidemic in the US and Europe is considered, with sections on the epidemiology of AIDS in women and children, the prevalence and incidence of HIV infection therein, and other surveillance approaches. Epidemic scope in Africa, the Caribbean, South America, Asia, and the Pacific is also addressed. Discussion also includes rates, risk factors, and mechanisms of transmission, as well as incubation period and clinical presentations of morbidity and mortality.